1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the same. In particular, the present invention relates to a zoom lens useful in an image pickup apparatus, such as a video camera, a digital still camera, a TV camera, or a silver-halide film camera.
2. Description of the Related Art
In recent years, an image pickup apparatus that uses a solid-state image sensor, such as a video camera, a digital still camera, a TV camera, or a silver-halide film camera, has a large number of functions. Furthermore, the size of the entire apparatus is small.
It is desired by the market, in a photographic optical system (zoom lens) used in such an image pickup apparatus, that the length of the entire photographic optical system is short, that the size of the photographic optical system is small, that the photographic optical system has a high zoom ratio, and that a high-resolution image can be captured in the entire zoom range. As a zoom lens that can respond to the desire like this, a rear focus type zoom lens is widely used.
More specifically, the rear focus type zoom lens executes focusing by moving a lent unit other than a first lens unit, which is disposed on the object side. In such a rear focus type zoom lens, the effective diameter of the first lens unit is usually smaller than the effective diameter of the first lens unit of a zoom lens that executes focusing by moving the first lens unit. Accordingly, it is easy to reduce the size of the entire zoom lens.
Moreover, the rear focus type zoom lens executes focusing by moving a small-sized lightweight lens unit. Accordingly, the necessary amount of driving force for driving the lens unit is small. Therefore, the rear focus type zoom lens can quickly focus on the object.
As the rear focus type zoom lens described above, a four-unit zoom lens has been marketed, which includes, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power.
U.S. Pat. Nos. 5,963,378 and 6,166,864 discuss a four-unit zoom lens that executes zooming by moving the second lens unit and executes focusing and the compensation of image plane movement caused by zooming by moving the fourth lens unit. In addition, U.S. Pat. No. 7,190,529 and Japanese Patent Application Laid-Open No. 2007-212537 discuss a four-unit zoom lens that executes zooming by moving each lens unit and executes focusing by moving the fourth lens unit.
U.S. Pat. No. 7,206,137 discusses a four-unit or five-unit zoom lens that includes, in order from the object side to the image side, a lens unit having a positive refractive power, a lens unit having a negative refractive power, a lens unit having a positive refractive power, and a lens unit having a positive refractive power. The zoom lens discussed in U.S. Pat. No. 7,206,137 executes zooming by moving the first through the fourth lens units and executes focusing by moving the fourth lens unit.
In order to reduce the size of a zoom lens, it is necessary to reduce the number of lens elements in the zoom lens while increasing the refractive power of each lens unit constituting the zoom lens. However, if a zoom lens has such a configuration the thickness of each lens unit may increase as the refractive power of each lens surface is increased. Accordingly, the length of the entire zoom lens cannot be sufficiently reduced. In addition, it becomes difficult to correct various aberrations, such as chromatic aberration, which may occur at the telephoto end.
Moreover, if the zoom ratio is increased, the degree of manufacturing error such as plane tilt of each lens element or each lens unit may increase. If the sensitivity to decentering of each lens element or each lens unit, the optical performance may greatly degrade due to decentering, and the optical performance during image stabilization may also greatly degrade. Accordingly, in a zoom lens, it is desired to reduce the sensitivity to decentering of each lens element or each lens unit to a level as low as possible.
In the above-described four-unit zoom lens and five-unit zoom lens, in order to achieve a high optical performance while achieving a high zoom ratio and a small-size zoom lens, it is important to appropriately set the refractive power and the lens configuration of each lens unit and a condition for moving each lens unit during zooming.
Particularly, it is important to appropriately set the refractive power of the first through the third lens units and the condition for moving each lens unit during zooming. If the above-described configurations are not appropriately set, it becomes difficult to implement a zoom lens whose total size is small, having a high zoom ratio, and having a high optical performance in the entire zoom range.